Update Ecto.Model.Schema docs
Fix for ecto.migrate task

Was throwing an error when executed. Talking to @josevalim on the google group and it turned out the fix is simply changing the "if" at line 45 to an "unless". Changing it resolved the error.
Merge pull request #228 from bryanjos/patch-1

Fix for ecto.migrate task
Fix migration task test
Merge branch 'master' into put-primary-key-readme
Update Ecto.Model.Schema moduledoc with notes on setting primary keys